Various types of materials for making dental impressions are known, but they are relatively inconvenient to use and can be unpleasant to the user and patient.
Dental plaster is one material used to make dental impressions. Gypsum, which is widely distributed naturally as calcium sulfate dihydrate (CaSO.sub.4.H.sub.2 O), has been used since 1844 in the production of dental plaster.
To form an impression using dental plaster, the gypsum is ground and subjected to temperatures of 110.degree. C.-120.degree. C. to provide large and porous crystals. These crystals typically require a 2:1 powder-gauging water ratio for proper consistency.
However, the preparation of an impression using such dental plaster is time-consuming and messy. Immediately prior to the time it is desired to make an impression, the gypsum crystals are mixed with an appropriate amount of water to produce a thin, fluid slurry. Subsequently, to form the impression or negative, the plaster slurry is placed in a tray, inserted into the mouth, pressed in place against the area in question, and held still until the plaster hardens at which time the plaster impression is removed from the mouth.
Generally, the impression plaster is used to obtain an impression or negative of the hard and soft tissues of the mouth. After the plaster impression is removed, model plaster may be poured into the impression to produce a cast (positive). A satisfactory impression plaster has a setting time from about 2.5-5.5 minutes, a setting expansion at 2 hours less than 0.15%, and a compressive strength at one hour between 8265.5-8845.5 psi.
Wax is another material used to make dental impressions. The wax compositions for dental usage are usually compounded in conventional melting and blending equipment and in a manner that avoids the degradation of wax properties. While soft, low melting, plastic waxes are suitable, when a complete impression is desired, hard high-melting, rigid base waxes are used with successively softer wax additions to build the complete impression.
Although the exact formula of an impression wax is typically a trade secret, several different components have been identified in the wax depending upon the intended use of the impression. For instance when a negative cast of the mouth structure is desired, paraffin, ceresin, vegetable wax, rosin, mastic gum, and spermaceti have been identified as components used in the impression wax.
When an impression wax is used to establish tooth articulation, i.e., the occlusion or horizontal relationship of the lower jaw to the upper jaw, high-flow, low-melting paraffins, microcrystalline waxes and resins have been identified as components of the impression or bite wax.
When an impression wax is used to detect tooth interference and high spots or improper fit of dentures bases, the wax is very soft and salve-like (i.e., a pressure indicating paste) and painted onto the tissue side of an impression or denture. When situated in the mouth the paste is forced out of the area having contact with the tissue.
At the time impression wax is used to obtain an impression, the wax may need to be heated. Alternatively, impression waxes, which are plastic and moldable at mouth temperature, must be cooled below mouth temperature to obtain a nonelastic mass which retains the impression. Thus, similar to impression plaster the preparation and use of impression waxes can be cumbersome and possibly dangerous to the patient, since the wax may be heated too much and burn the inside of the mouth or may not be soft enough to create an impression with an easy bite and thereby cause an injury to the mouth, teeth or jaw.
Impression materials based on reversible hydrocolloids (agar), irreversible hydrocolloids (alginates), combinations of agar and alginate, and to a lesser extent, oxide-eugenol cements, have also been employed to make dental impressions.
Agar-based impression materials are thermally reversible, aqueous gels that become viscous fluids in boiling water and set to an elastic gel when cooled below 35.degree. C. Such material is typically used to obtain impressions of inlay and crown preparations and gingival areas by filling the preparation or area with the impression material injected from a hypodermic syringe.
To form agar-based impression material, about 6-12% agar is generally used with a 75-80% water content. Fillers such as zinc oxide and clays may be incorporated as well as other additives such as boron incorporated as well as other additives such as boron compounds (borax, calcium metaborate and organic borate compounds), waxes or fatty acids, emulsifying agents and plaster-accelerating agents (i.e., potassium sulfate, magnesium sulfate and zinc sulfate). To remain stable in storage agar impression materials must be free of salts or additives that crystallize and salt out, degrade the complex molecule or induce syneresis in the agar gel.
Moreover, prior to insertion or injection into the mouth, agar-based materials require time consuming preparation which includes cooling the material to produce a gel. After a gel has been obtained and inserted in the mouth or over an area for which an impression is desired, a setting time is required to elapse before the gel can be removed.
Potassium or sodium salts of alginic acid are useful alginate impression materials. Alginate-based irreversible hydrocolloid impression materials, which constitute chemically reactive mixtures, are supplied as a dry powder which is mixed with water to form a viscous but slightly fluid mass (gel or paste). In use the gel or paste is placed over the area of which an impression is desired and allowed to set (about 1-4 minutes) to provide a strong, tough elastic gel. The impression is removed in a swift motion to minimize any tearing or distortion.
However, the agar- and alginate-based impression materials lack dimensional stability with any loss or gain of water. Consequently, improved systems are sought.
Accordingly, nonaqueous, polysulfide-, condensation silicone-, addition silicone- and polyether-based systems have also been employed as impression materials.
Polysulfide impression materials also tend to be messy and cumbersome in use since polysulfide materials are generally supplied as a two-part paste system--a polysulfide-containing, liquid polymer base and a setting-agent paste (i.e., lead peroxide). When the liquid polymer base and paste are mixed together (in approximately equal amounts) a homogeneous, streak-free mass (elastic solid) is provided. The mass is transferred to a tray which is then placed over the area of interest and held motionless (for about 5-10 minutes) until the rubber has set.
The polysulfide base material ordinarily contains 50-80% of the polyfunctional mercaptan and, additionally, a filler (i.e., calcium carbonate, alumina, silica), diluents, modifying agents, retarding agents and buffering agents.
The setting agent paste typically comprises alkalies, sulfur, metallic oxides, metallic peroxides, organic peroxides or metal-organic salts. The paste can also include a diluent, filler, buffering agent or other modifier.
The silicone impression material systems are nontacky and can be wiped away from instruments or hands at any stage of the mix or set. However, the silicone systems are moisture sensitive and subject to deterioration when exposed to the atmosphere. The silicone impression materials provide impressions in a manner similar to the polysulfide systems.
Condensation silicone impression materials are two part systems based on hydroxyl-terminated polydimethylsiloxane. The systems can be either two pastes or a paste-liquid catalyst which must be mixed outside of the mouth. The silicone is a viscous liquid to which colloidal silica or micronized metal oxides are added to provide a paste. The catalyst part of the system can be a tetraalkyl silicate containing 50% ethoxy group, such as tetraethyl orthosilicate and 1-2% of an organic tin activator, which is not necessarily desirable for placement in the mouth.
Addition siloxane impression materials are supplied in an awkward two-paste systems wherein one paste contains low molecular weight silicone with terminal vinyl groups and reinforcing filler and the second paste contains a hydrogen-terminated siloxane oligomer, filler and chloroplatinic acid catalyst. Mixing the two pastes provides a cross-linked elastomer. Since hydroxyl-containing silicones evolve hydrogen, palladium has been added to some formulations.
Polyether impression material is also supplied in a two-paste system. Typically one paste contains a branched polyether molecules having a main chain of an ethylene oxide-tetrahydrofuran copolymer and a second paste containing an aromatic sulfonate ester catalyst. When mixed together cross-linking is brought on by cationic polymerization via the imine end groups. The polyether impression system is used similar to the silicone and rubber systems. Although setting time is less than 2.5 minutes, the polyether elastomer is more difficult to remove from the mouth, will readily tear, and has poor dimensional stability in water.
Each of the impression materials described above requires mixing of different materials and/or temperature fluctuation to provide the impression material which will be used to provide an impression of the teeth. Thus, there remains a need for a formulation of impression material having a fairly stiff yet malleable consistency which is simple to use and does not require preparation time or setting time. There is a particular need for impression materials which are easily handled and accurately reproduce or register the dimensions, surface details, and interrelationship of the teeth.
There is also a need for an impression material which can retain its desirable flexibility, and its capability to take and retain a dental impression, for a prolonged period of time without suffering a change in these properties during storage.